There has been increasing interest in the use of integrated optical systems for optical recording technology because waveguide systems have several advantages over conventional optical systems including reduced size and weight, component alignment as part of fabrication, integration of optical elements with detectors and electronics on the same substrate, and processing techniques. Integrated optical system have been designed to serve as transmitters to deliver light to an optical disk, or receivers for processing information contained in the beam returning from the optical disk. Waveguide gratings have been the primary element for input and output coupling light considered for transmitters and receivers. When used as receivers, waveguide gratings can be designed to sample different areas or attributes of the return beam and perform multiple functions to contribute to signal processing (i.e., focusing of input coupled beams, beam splitting, polarization selective coupling, etc.).
One early concept for utilizing integrated optics to perform transmitter and receiver functions is disclosed in U.S. Pat. No. 4,779,259. Disclosed is a grating used to focus an optical beam exiting from a waveguide onto an optical disk using a single grating element having a curvature and chirping in the design of the grating grooves. The returning light was received through the same element and two gratings in the waveguide optical path were used to divide the guided beam to obtain focus error information. U.S. Pat. Nos. 4,833,561, 4,868,803 and 4,885,732 disclose concepts for use of gratings, designed to be only a receiver, having three sections to detect signals from a magneto-optical disk. The center section of the grating differs from the outer sections by the groove pitch and a resulting sensitivity to TM polarized light rather than TE polarized light at the incident angle of the return beam. Each of the gratings has curved grooves to focus the three beams within the waveguide for optical processing of the focus error and reduced detector size. Similarly, U.S. Pat. No. 4,876,680 discloses sectioning a grating by the orientation of the grating grooves as a novel method of detecting tracking error signal. These patents are exemplary of the use of gratings as input couplers and optical elements to detect focus errors based on the pupil obscuration method.
U.S. Pat. No. 4,672,187 discloses a device which performs as both transmitter and receiver with separate input and output coupling gratings. The receiver grating detects focus errors based on the coupling efficiency of the return beam. Unfortunately, this device is limited in its application because it does not allow for orthogonal detection of the tracking and focus error signals. Also, there is no optimization of the grating pattern in the area of the beam to optimize the signal generated for servo operation in an optical head. Accordingly, it will be appreciated that it would be highly desirable to have an optical head that facilitates orthogonal detection of the tracking and focus error signals, and optimizes the grating pattern in the area of the beam to maximize the signal generated for servo operation.
FIG. 1 is a schematic diagram of a conventional write-once optical recording head 10 for writing on a recording layer 12 of an optical disk 14. The apparatus comprises a diode laser 16 which emits radiation that is collimated by a lens 18 and directed to a circularizing prism 20 and polarizing beam splitter 22. The beam encounters quarter wave plate 24 before an objective lens and actuator 26 on the way to the recording layer 12 of the disk 14. From the polarizing beam splitter 22 the beam returning from the disk goes to a second beam splitter 28 where a portion of the beam is directed to tracking error and data detectors 30 by processing lens 32. The other portion of the beam is directed to focus error detectors 34 by processing lens 36 and knife edge 38. The optical head profile is indicated by a dotted line 40. This device is not as limited in its application and does allow for orthogonal detection of the tracking and focus error signals. It does require precise alignment of the optical components and is bulky. Accordingly, it will be appreciated that it would be highly desirable to have an optical head that facilitates orthogonal detection of the tracking and focus error signals that is compact in size and does not require precision alignment of the detectors located at the focal position of the processing lenses 32 and 36.